Magnetic recording medium with two storage layers for recording different signals



June 27, 1967 s. H. MAY 3,328,195

MAGNETIC RECORDING MEDIUM WITH TWO STORAGE LAYERS FOR RECORDINGDIFFERENT SIGNALS Original Filed Nov. 30, 1962 READBACK VOLTAGEHILLIAMPS WRITE MAGNETOMOTIVE FORCE AMPERE TURNS INVENTOR GORDON H. MAY

BY RM 8, 6M

ATTORNEY United States Patent 3 Claims. (Cl. 117-69) This application isa continuation of application Ser. No. 241,196, filed Nov. 30, 1962, nowabandoned.

The present invention relates to a magnetic recording medium for datastorage and more particularly to a magnetic recording medium which has agreatly increased magnetic data storage capacity.

In magnetic data storage devices, one or more trans- (lucers and astorage medium are supported in proximity to each other for relativemovement. Electrical signals applied to the transducer are stored in thestorage medium in the form of minute areas of magnetic fluxconcentration. Each storage medium has a fixed available surface area inwhich data may be recorded and the capacity of which, for a given recorddensity, remains constant. The continuing demand for larger storagecapacities in magnetic data storage devices has heretofore been met byincreasing the available surface area of magnetic medium within thedevices. This approach has successfully produced an increase in storagecapacity, but only with a corresponding increase in the bulk of theoverall device.

The object of the present invention is to provide a magnetic recordingmedium having approximately double the usual storage capacity withoutany coresponding increase in surface area or bulk.

The above object is realized in the present invention by provision of adual magnetic-layer medium in which two distinct layers of magneticmaterial are superimposed on a non-magnetic substrate. With this medium,different sets of data can be recorded in the two layers toapproximately double the storage capacity of the medium.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of a preferred embodiment of the invention, as illustratedin the accompanying drawing wherein;

FIG. 1 is a partial elevation view in section of a mag netic recordingmedium according to the present invention, and

FIG. 2 is a plot of the magnetic characteristic of a typical recordingmedium according to this invention.

As shown in FIG. 1, the magnetic recording medium of the presentinvention includes a supporting substrate 11 and two distinct layers, 12and 13, of magnetic material of difierent coercivities superimposed onthe substrate. The substrate 11 is of non-magnetic material, such asaluminum, brass, etc., While upper layer 12 and lower layer 13 are ofany suitable material such as nickelcobalt, magnetic iron oxide, etc.With this construction, one set of signals can be recorded in the upperlayer 12 and a different set of signals recorded directly underneath inthe lower layer 13. To make this dual layer recording elfective, the twosets of signals must be individually recognizable and one must existindependently of the other. To be individually recognizable thefrequencies at which the two sets of signals are recorded must besufiiciently distinct to allow the two to be separated by filtering.This requirement can be met by recording the set of signals in the upperlayer at a high frequency, approximately three or four times thefrequency of the other set of signals recorded in the lower layer. Inaddition, so that one set of 3,328,195 Patented June 27, 1967 signalsmay exist independently of the other, the coercivity of the lower layermust be considerably greater than that of the upper layer. This is toallow the set of signals recorded on the lower layer to remainundisturbed by subsequent writing and rewriting of the set of signals inthe upper layer. The particular ratio of coercivities between the twolayers will depend to some degree upon the types of information to berecorded, since the greater the ratio of coercivities the more indeliblethe lower layer becomes. In a given case, the minimum effective ratio isapproximately 5 to 1 whereas 8-10 to 1 would be a nominal ratio.

Referring to FIG. 2 of the drawing, the saturation characteristic of atypical dual layer disk has been determined by writing with square wavecurrent of varying amplitude at a constant frequency and then measuringthe amplitude of the readback voltage. The medium tested included anupper layer 30 microinches thick with a coercivity of oersteds while thelower layer was microinches in thickness with a coercivity of 1000oersteds. As shown on the graph, the upper layer of the medium saturatedat approximately 1.2 ampere turns While the lower layer saturated atapproximately 6.75 ampere turns. Alinear region of the lower layer isfound between 1.5 ampere turns and 6.0 ampere turns. With proper D.C.biasing a low frequency signal may be linearly recorded in the lowerlayer while a frequency signal is saturate recorded in the upper layer.The lower layer should be written at a frequency or at a band offrequencies whose upper limit is well below the lowest frequencycontained in the upper layer. The saturation characteristics of theupper and lower layers can be used to determine the write and erasecurrents which would have a minimum effect on the information recordingof the lower layer.

The low frequency information to be recorded in the lower layer may bewritten during manufacture of the medium prior to the application of theupper layer. This would be particularly advantageous in the case wherethe low frequency information was meant to be permanently recorded. Ifit is desired however, the low frequency information may be writtenthrough the upper layer after the medium is manufactured. In this case,a large magnetomotive force, i.e., in the order of 5 ampere turns forthe transducer and lower layer coercivity tested above, would benecessary for this purpose.

The present invention is applicable to various type magnetic recordingmedia such as a drum, disk tape, etc. The layers of magnetic materialmay be applied by any suitable process, eg electroplating, spraying,painting, etc.

While the invention has been particularly shown and described withreference to a preferred embodiment thereof, it will be understood bythose skilled in the art that the foregoing and other changes in theform and details may be made therein without departing from the spiritand scope of the invention.

What I claim is:

1. A magnetic data recording medium comprising:

a non magnetic substrate, and

means for storing simultaneously a plurality of different sets ofmagnetic information signals, said means in- Cluding a first signalstorage layer of high coercivity magnetic material applied directly onthe substrate for storage of a first set of relatively low frequencymagnetic signals, and a second signal storage layer of low coercivitymagnetic material applied directly on the first layer for storage of asecond set of relatively high frequency magnetic signals, the coercivityof the first layer being a minimum of five times that of the secondlayer.

2. A magnetic data recording medium as set forth in claim 1 in which thefirst signal storage layer is con- 3 4 siderably thicker than the secondsignal storage layer and 3,131,078 4/ 1965 Fuller et a1 117-8 thefrequency of the second set of magnetic signals is 3,171,754 3/1965Smaller 117-71 approximately three times that of the first set.3,185,971 5/ 1965 Brette et al 346-74 X 3. A magnetic data recordingmedium as set forth in 3,219,353 11/1965 Prent-ky 274-414 claim 1 inwhich the ratio of coercivities of the two 5 3,255,033 6/1966Schmeckenbecher 117-47 signal storage layers is normally 8-10 to one.3,268,353 8/1966 Melillo 117-69 OTHER REFERENCES Soohoo, Ronald R:Magnetic Thin Films, Harper & Row, NY. (1965), QC 161 S 650.2 Pages143-5.

References Cited UNITED STATES PATENTS 2,643,130 6/ 1953 KorneiScientific Report No. 1., Contract AF 19 (604)-4978, 2,647,954 8/ 1953Howell 179-1002 Thin Ferromagnetic Films (1960), pages 9-10. 2,691,07210/1954 Mathes 179-1002 2,941,901 19 0 rin et 1, 117 7 WILLIAM H MARTIN,Primary Examiner.

3,052,567 9/1962 Gabot et a1 117-76 W. D, HERRICK, Assistant Examiner.

1. A MAGNETIC DATE RECORDING MEDICUM COMPRISING: A NON MAGNETICSUBSTRATE, AND MEANS FOR STORING SIMULTANEOUSLY A PLURALITY OF DIFFERENTSETS OF MAGNETIC INFORMATION SIGNALS, SAID MEANS INCLUDING A FIRSTSIGNAL STORAGE LAYER OF HIGH COERCIVITY MAGNETIC MATERIAL APPLIEDDIRECTLY ON THE SUBSTRATE FOR STORAGE OF A FIRST SET OF RELATIVELY LOWFREQUENCY MAGNETIC KSIGNALS, AND A SECOND SIGNAL STORAGE LAYER OF LOWCOERCIVITY MAGNETIC MATERIAL APPLIED DIRECTLY ON THE FIRST LAYER FORSTORAGE OF A SECOND SET OF RELATIVELY HIGH FREQUENCY MAGNETIC SIGNALS,THE COERCIVITY OF THE FIRST LAYER BEING A MINIMUM OF FIVE TIMES THAT OFTHE SECOND LAYER.